Viral hepatitis is the single most important cause of liver disease. Many infectious agents, including hepatitis A, B, C, D, and E viruses, can cause viral hepatitis. The Hepatitis B virus (HBV), for example, is a small, enveloped DNA virus that infects 400 million people worldwide. HBV is unusual among DNA viruses because its replication involves reverse transcription of an RNA intermediate. Infection with HBV induces a broad spectrum of liver diseases, including acute hepatitis (that can lead to fulminate hepatic failure) as well as chronic hepatitis, cirrhosis, and heptocellular carcinoma (HCC). There is an effective preventative vaccine, however, 316,000 new cases of HBV-associated cancers are still diagnosed each year. WHO, World Health Report 1996: Fighting Disease, Fostering Development (World Health Organization, Geneva, 1996).